I haven't come up with anything in CB for a while, but i thought i'd bring this one up that came to mind.

Berserker's Rage
Initial Training Cost: 840
The Rage of the Berserker fills the minion with unholy strength, boosting strength by 5% during ranged rounds and 10% during melee. This strength is maintained throughout the fight, the strength added is calculated upon the minions Pre battle stats (their trained strength)

BR has reduced effect when its level is less than 1/5 of the minions strength.
The maximum effect occurs when BR is trained to 1/5 of the minions strength (not including Giant Strength)

I think this would be at least a moderately practical skill, though i am aware that even if it was accepted, it would be unlikely, at best, to be implemented in the game.
That being said, i would like people to discuss the idea of this.

One thing that I might change is have the strength % bonus increase per round.

So start it at 2% for round one then go to 4%, 6%, ect. This would leave you with a total of 2550% of increased strength at the end of the 50 rounds.

This might seem like a lot, but considering BL gives 75% increased damage. Taking this to be close to double damage, which it takes 4 times your strength to double your damage, then it would take around round 20 before reaching the same damage level. However you would do way less before then and way more afterwards.

I see this as counter-exbow skill, as exbow drains, you gain, exbow drains, you gain, ... so that's definitely different to BL as with bl your str might be at 0 in round 3 and with this you gain every round new strength

one question: does exbow drain below zero str ? would this skill have to catch up from below zero str or would id always start from 0 ?

Exbows quite regularly drain below 0 str and this skill would counter EC far more effectively than it would the exbow.
I quite like your suggestion of an increasing % EoD, and i think it would suit the skill far more than my suggestion. In fact, having EoD's suggestion would make it far more practical when it comes to returning a minion to positive strength. My only concern is whether it can be coded as easily as my suggestion (i don't know which is easier from that view, though mine is more static for that reason).

a tattoo that boosts Amf! :D anyway; in the game, i hardly seen any battle go up to 25, only ones that i've seen to reach 25+ was RoBF vs wall. a tank would hardly be useful there if your damage will be RoBF. i believe the boost can be a bit higher? :P or something that grows with rounds, like each round adds a multiplier of 0.025 in ranged and 0.5 in melee;
2 M str minion
ranged round 1 multiplier is 1.025
ranged round 2 multiplier is 1.05
ranged round 3 multiplier is 1.075
ranged round 4 multiplier is 1.1
ranged round 5 multiplier is 1.2
melee round 1 multiplier is 1.25
melee round 2 multiplier is 1.3
melee round 3 multiplier is 1.35
long post ._.

I love how i don't mention this as a counter to Exbow nor as a replacement for BL and its seen as both.
This is simply a skill idea i had i felt like sharing, and EoD is entirely correct about its usage. Its never gonna be a hard counter for the exbow, in fact i wouldn't ever use it solely for that purpose. And BL is infinitely more effective early in the battle. This would be best used during a long fight, and its been pointed out that there really isn't many of those.
Its usable and its useful if a minion were based around it, but it can't be used by an archer, an UC user or in conjunction with BL. I just wanted to provide additional game options and variety.